commerce: add provider-neutral payment-request envelope to Phase 5 backlog

Carries {:order :amount :currency :return-url} on the 'payment suspension so any
provider's host adapter can initiate payment without the engine knowing the
vendor; order-settle!(ref, amount) stays the vendor-neutral resume seam.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plans/commerce-on-sx.md

@@ -89,6 +89,12 @@ that unlocks the most tests per effort each iteration.
   flow-on-sx flow, recorded in the ledger; idempotent.
 - [ ] stock-constrained reservation — order-begin! fails (railway `fail`) when
   requested qty exceeds stocko availability; reservation decrements a stock view.
+- [ ] provider-neutral payment-request envelope — the `'payment` suspension carries
+  `{:order :amount :currency :return-url}` (mirroring flow `host.sx`'s `request`
+  envelope) so any provider's host adapter can initiate payment without the engine
+  knowing the vendor. SumUp/Stripe/etc. adapters stay at the IO edge (orders
+  service); `order-settle!(ref, amount)` remains the vendor-neutral resume seam.
+  Keeps lib/commerce provider-agnostic; enables multi-provider support in the core.
 
 ## Progress log
 - 2026-06-07 — `recon.sx` + `federation.sx` (**Phase 4 complete — roadmap done**).